Multi-layered holographic read-only memory and data retrieval method

ABSTRACT

An inexpensive multi-layered holographic read-only memory having a large capacity is provided. In the memory in which single-mode slab waveguides are multi-layered, a periodic scattering center whose period approximately agrees with the period of the guided mode is provided in at least one of a core layer and a clad layer in each waveguide so that a guided wave in the waveguide is diffracted by the periodic scattering center to the outside of the waveguide and a holographic image is generated.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a multi-layered holographicread-only memory, preferably used as a mobile (or portable) memory cardsuch as a magnetic card and an IC card. This type of card is difficultto forge or reproduce, and thus can be used as an authentication cardfor electronic commercial transaction. Additionally, the present memoryhas a large capacity and can be manufactured at low cost, and thus issuitable for distributing software for music, pictures, computerapplications, or the like.

[0003] This application is based on Patent Applications Nos. Hei10-32578, Hei 10-44941, and Hei 10-75336 filed in Japan, the contents ofwhich are incorporated herein by reference.

[0004] 2. Description of the Related Art

[0005] Magnetic cards such as a telephone card are conventionally usedas mobile (or portable) information cards which a user can carry in apocket. Recently, IC cards have become practical to use, and applicationof the IC cards to the electronic commercial transaction has beeninvestigated. The magnetic cards are cheap, but may be forged. The ICcards are difficult to forge, but their cost per bit is expensive.

[0006] The holographic storage technique is another technique applicableto a data memory which is difficult to forge and has a large capacity.Holography can be classified into thin film holography and volumeholography.

[0007] A volume holographic memory has a larger storage capacity;however, no data duplication technique suitable for the volume hologramexists. Therefore, this type does not suit mass production, and it isdifficult to apply the volume holography to a read-only memory used asan authentication card or used for distributing software applications.

[0008] A thin film hologram can be mass-produced using a printingtechnique, but has a limited memory density. Therefore, in considerationof the size and convenience of a necessary data retrieval device, thememory using thin film holography has less appeal in comparison with theIC card. Even if such thin film holograms are multi-layered so as tosolve the above problem and to increase the storage capacity,holographic images, reconstructed from each hologram by using anordinary reconstruction method, are simultaneously reconstructed.Therefore, due to the crosstalk being large, necessary data orinformation cannot be obtained.

SUMMARY OF THE INVENTION

[0009] An objective of the present invention is to provide aninexpensive multi-layered holographic read-only memory having a largecapacity, which is applicable to a mobile card or a storage medium usedin a data storage device. Another objective is to provide amulti-layered holographic read-only memory by which the data-retrievalspeed can be improved.

[0010] In order to realize the above objective, the present inventionprovides a multi-layered holographic read-only memory in whichsingle-mode slab waveguides are stacked to be multi-layered, wherein aperiodic scattering center whose period approximately agrees with theperiod of the guided mode is provided in at least one of a core layerand a clad layer in each waveguide so that a guided wave in thewaveguide is diffracted by the periodic scattering center to the outsideof the waveguide and a holographic image is generated.

[0011] That is, the principle of the thin film holography is used, andthe hologram based on this is difficult to forge while it can bemass-produced. In the present invention, such thin film holograms aremulti-layered, and the holographic image of each layer can beindividually reconstructed.

[0012] In the general thin film holography, even if the incidentdirection or wavelength of a beam for reconstructing the image (i.e., areference beam) is changed so as to change the position, magnification,or diffraction direction of the reconstructed image, the reference beamis always diffracted. That is, in the multi-layered thin film holograms,as far as the reference beam reaches each thin film hologram, crosstalkis inevitable regardless of the incident direction of the referencebeam. In the present invention, each thin film hologram is embedded inthe waveguide and the guided wave functions as a reference beam, therebypreventing the reference beam from reaching holograms other than thetarget hologram.

[0013] Therefore, in the present invention, the data storage capacitycan be increased like the volume holographic memories while theprinciple of the thin film holography suitable for mass production canbe used. Accordingly, an inexpensive read-only memory having a largerstorage capacity can be realized. If the present memory is applied to aportable memory card, a rotating mechanism employed in the optical discor the like is unnecessary; thus, power necessary for the retrievaldevice (for retrieving musical or video data) can be reduced. If thepresent memory is applied to an authentication card, the card isdifficult to forge and various additional information data can also bestored therein. Therefore, convenience can be improved.

[0014] Typically, at least one of the edges of the multi-layered slabwaveguide is cut so as to produce a reflecting surface which is slanted(or inclined) by approximately 45° with respect to the normal directionof the waveguide plane, and light is incident from the directionsubstantially perpendicular to the waveguide plane on the reflectingsurface so as to introduce the light into the waveguide. If themulti-layered slab waveguide has opposite edges, both the edges mayfunction as 45°-cut reflecting surfaces, and light-introducing (orcoupling) positions can be determined such that guided waves incidentfrom these edges do not overlap with each other in the relevantwaveguide plane. Furthermore, a plurality of the multi-layered slabwaveguides may be placed and bonded with each other in the waveguideplane so as to make a card.

[0015] According to such variations, possible areas for storing readabledata as holograms can be increased; thus, the storage capacity can beincreased, and the data retrieval speed can be improved. If the presentinvention is applied to a portable card, the effective storage area canbe increased depending on the total area of the card.

[0016] On the other hand, the multi-layered slab waveguide may have adisc shape, and a light-introducing section may be provided in a centralarea of the disc, so as to guide light towards the outer circumferenceof the disc.

[0017] Typically, the light-introducing section is a reflecting surfacehaving the shape of a 45°-slanted side face of a cone, and light can beincident from the direction substantially perpendicular to the discplane on the reflecting surface so as to introduce the light into thewaveguide.

[0018] In this case, the following arrangement is possible: a pluralityof coupling points for introducing light are concentrically andperiodically placed in the reflecting surface; a guided wave istransmitted from each coupling point towards the outer circumference ofthe disc while the guided wave expands as a fan-shape having apredetermined angle of expansion; and the predetermined angle ofexpansion is determined in order that fan-shaped portions correspondingto each coupling point do not overlap with each other.

[0019] In the above arrangement, the coupling points are circularlyconcentrated near the center of the disc. Therefore, with a single headabove the circumference of the circle of the coupling points, thewaveguide of each fan-shape portion can be accessed in turn by rotatingthe disc, and it is efficient.

[0020] Generally, a rotational data-retrieval device has a much simplerstructure and a higher access speed in comparison with a linearmechanical-motion data-retrieval system. When the target layer ischanged, the lens of a light source must be moved also in this case(like the card type). However, in the rotational data-retrieval device,the requirement for the necessary moving distance is 1 mm at the most. Aprecise micro-motion mechanism (i.e., actuator) necessary for such astroke is widely used for optical discs or the like, and is notexpensive. In addition, such a micro-motion mechanism has a shortresponse time of approximately 1 ms.

[0021] When data stored in the disc memory are retrieved by rotating thedisc, it is possible to extract a non-diffracted portion of the guidedwave outside of the memory so as to establish a synchronous condition inthe rotation.

[0022] Consequently, according to the present invention, both thepossible data-storage area of the hologram and the data-retrieval speedcan be remarkably improved.

[0023] Japanese Patent Application, First Publication, No. Hei 9-101735discloses a data retrieval technique using multi-recorded holograms. Inthis case, a part of or the whole of the holograms is made of an opticalrecording material, and data are recorded using the opticalinterference. Data are retrieved using a reference beam. However, everyobject beam passes through all the layers during the recording of eachhologram. Here, exposure of a specific intensity must be performed so asto obtain the necessary signal intensity. In this method, the S/N ratiois decreased in inverse proportion to the square of the number oflayers. In contrast, in the present invention, the hologram is providedin advance for each target layer, and data are retrieved using theguided wave, as described above; thus, the S/N ratio is merely decreasedin inverse proportion to the number of layers. That is, Hei 9-101735 hasdifferent structure and functions in comparison with the presentinvention, and has an essential problem with respect to themulti-layered structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a diagram explaining the structure of the multi-layeredholographic read-only memory card and the method for inputting andoutputting a light beam in the first embodiment according to the presentinvention.

[0025]FIG. 2 is a diagram explaining the periodic scattering center ofthe present invention.

[0026]FIG. 3 is a diagram explaining example processes of manufacturingthe multi-layered holographic read-only memory card as an example in thefirst embodiment according to the present invention.

[0027]FIGS. 4A to 4C are diagrams explaining the following processes ofmanufacturing the multi-layered holographic read-only memory card in thefirst embodiment

[0028]FIG. 5 is a diagram explaining example processes of manufacturingthe multi-layered holographic read-only memory card as another examplein the first embodiment, by employing the patterning method using a UVcurable resin.

[0029]FIGS. 6A to 6C are diagrams explaining the following processes ofmanufacturing the multi-layered holographic read-only memory card in thefirst embodiment.

[0030]FIG. 7 is a diagram explaining the traveling state of a guidedwave.

[0031]FIG. 8 is a diagram showing an arrangement in which fan-shapedwaveguides are most closely arranged.

[0032]FIG. 9 is a diagram showing a card manufactured by placing blocksin the waveguide plane.

[0033]FIG. 10 is a diagram showing another card manufactured by placingblocks in the waveguide plane.

[0034]FIG. 11 is a diagram explaining a method of retrieving data byusing a plurality of coupling lenses and light-receiving elements.

[0035]FIG. 12 is also a diagram explaining a method of retrieving databy using a plurality of coupling lenses and light-receiving elements.

[0036]FIG. 13 is a diagram showing the structure of the holographicmemory card as an example in the second embodiment.

[0037]FIG. 14 is a diagram showing the structure of the holographicmemory card as another example in the second embodiment.

[0038]FIG. 15 is a diagram explaining the disc memory and the dataretrieval method in the third embodiment.

[0039]FIG. 16 is a diagram explaining the unit for storing data in thememory of the third embodiment.

[0040]FIG. 17 shows the disc memory as an example in the thirdembodiment.

[0041]FIG. 18 is a diagram explaining an example synchronous method whenretrieving data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] Hereinbelow, embodiments according to the present invention willbe explained with reference to the drawings; however, the presentinvention is not limited to the embodiments, and various variations andmodifications are possible.

[0043] First Embodiment

[0044]FIG. 1 is a diagram explaining the structure of the multi-layeredholographic read-only memory card and the method for inputting andoutputting a light beam in the first embodiment.

[0045] As shown in FIG. 1, this card has a periodic structure consistingof “cladding 11-1/core 12-2/cladding 11-2/core 12-2/ . . . /cladding11-n”. Each unit of “cladding/core/cladding” functions as a single-modeslab (or thin film) waveguide at the wavelength of laser 13 to be used.The core of this slab optical waveguide is made of a transparentmaterial such as quartz or polymeric materials, and has a plate shape,and each core is put between materials having a lower refractive index.In this structure, light can be confined inside the core and betransmitted in a direction within the relevant plane; thus, thiswaveguide can be applied to a component used in optical communication.That is, in the multi-layered holographic read-only memory card, suchslab waveguides are stacked to be multi-layered and each waveguide layercomprises a hologram, as explained later.

[0046] Here, reference numeral 14 indicates a convex lens; however, acylindrical lens may be used. At least one of the edges of themulti-layered slab waveguide is reflecting surface 15, the angle ofwhich, with respect to the direction perpendicular to the waveguideplane (i.e., the normal direction), is 45°. The position of convex lens14 is adjusted so that the focus of the retrieval laser beam 13 ispositioned at the 45°-cut core portion of a target waveguide.

[0047] If the reflecting surface 15 is exposed, the total reflection isobserved, and no particular reflective layer is necessary. However, ifthe reflecting surface is protected using a resin or the like so as toproduce resistivity, a dielectric or metallic layer should be formed asa reflective layer.

[0048] In order to couple laser beam 13 to the waveguide, the NA(numerical aperture) of the convex lens 14 should be smaller than orequal to that of the waveguide.

[0049] However, with a smaller NA, the focal spot of the lens is larger.Therefore, in the case of using a single-mode waveguide, if a laser beamis directly focused from the air onto the waveguide, the spot size isalways larger than the width of the waveguide, and the couplingefficiency of 100% cannot be achieved.

[0050] The NA of the lens (i.e., NA_(L)) is defined as follows:

NA _(L) =D/{square root}(f ² +D ²)

[0051] where the diameter of lens is 2D, and the focal length of lens isf, and the NA of the waveguide (i.e., NA_(WG)) is defined as follows:

NA _(WG)={square root}(n _(a) ² −n _(c) ²)

[0052] where the refractive index of the core is n_(a), and therefractive index of the cladding is n_(c).

[0053] As shown in FIG. 1, the light beam incident from the reflectionpoint 18 onto the waveguide is converted to guided wave 16, whichtravels mainly within the core of the waveguide while the light beamexpands as a fan-shape, the pivot of which is the reflection point (thecoupling point of the guided wave) 18. Here, the angle of the expansionis “2 sin⁻¹ (NA_(L))” which is variable according to the kind of convexlens 14. If a cylindrical lens is used as the lens 14, the guided wavetravels with a fixed width, and does not expand as a fan-shape.

[0054] The guided wave 16 is partially scattered due to a scatteringcenter (here, hologram) provided in the core or clad layer, and thescattered portion escapes outside the waveguide. If scattering center 19has periodicity, there is a direction in which the scattered light willbe in phase, and in turn, this scattered light is referred to asdiffracted light 17 traveling in this direction. That is, the light isalso diffracted outside the waveguide, thereby generating holographicimage 20. This holographic image can be detected using a two-dimensionaldetector such as a CCD (charge coupled device) so that (information)data are retrieved. In addition, by shifting the convex lens 14 in FIG.1, the target wave-guiding layer can be changed; thus, data stored in aselected or target layer can be retrieved.

[0055] In FIG. 1, if the scattering center is provided at the interfacebetween the core land clad layers, or in one of these layers, in eithercase the diffracted light is output in both upward and downwarddirections.

[0056] Actually, the above periodic scattering center is a periodicrough pattern or a periodic modulation of the refractive index existingin the interface between the core and clad layers. However, another typeof the periodic scattering center for partially leaking the guided wavemay be used. As a practical example, the refractive index, absorptance,or thickness of the core is modulated in accordance with the hologrampattern, and the easiest method among these is the modulation of thethickness. For example, a previously-calculated concavo-convex patternfor fabricating a desired hologram is formed on a master plate made of ahard material such as metal, and the concavo-convex pattern istransferred to a plastic polymeric sheet by employing the above masterplate and using a printing technique. This plastic polymeric sheet isused as a core and a cladding. Accordingly, mass production of mediastoring the same contents (or information) is possible, as theconventional media such as CDs.

[0057] In order to reduce crosstalk, it is important that thediffraction efficiency with respect to the hologram is set to be small,such as 0.1%. As another method, a UV (ultraviolet) curable resin isused as a material for the core or cladding, and pattern irradiationusing the UV is performed. It is also possible to draw a periodicrefractive-index modulation pattern by using the electron impingement.

[0058] The following is an explanation of the parameters such as therefractive index and the thickness of each layer, used for implementingthe above-described concepts for the light coupling to the waveguide,the light traveling in a single-mode waveguide, and the generation of anholographic image. Here, λ indicates the wavelength of laser beam 13,n_(a) and d_(a) respectively denote the refractive index and thethickness of the core, and n_(c) and d_(c) respectively denote therefractive index and the thickness of the cladding. It is necessary toselect a material of a small absorptance for both core and cladding.

[0059] The condition necessary for a single-mode waveguide is indicatedby the following formula (1).

d _(a)<λ/2{square root}(n _(a) ² −n _(c) ²)   (1)

[0060] If Polymethyl methacrylate, that is, PMMA (n_(a)=1.492) isselected for the core, a UV curable resin (n_(c)=1.480) is selected forthe cladding, and a semiconductor laser of wavelength (A) of 680 nm isused as a light source, then the thickness of each core must be smallerthan 1.8 μm.

[0061] When the above refractive indices are selected, NA_(WG), the NAof the waveguide, is 0.189. If NA_(L) (the NA of the lens) is set to bethe same value, that is, 0.189, then the spot size of the focus is 4.3μm, which corresponds to the diameter of the Airy disc “1.2λ/NA_(L)”.This value is larger than 1.8 μm, that is, the thickness of the corelayer; thus, the coupling efficiency is (i) 71% in the case using acircular convex lens, or (ii) 77% in the case using a cylindrical lens.

[0062] Hereinbelow, the directional coupling between layers in themulti-layered waveguides will be explained. It is assumed that pairs of“cladding/core” are periodically multi-layered, and a light beam isinput only into the j-th layer so that the beam travels in the zdirection. While the light beam is wave-guided in the z direction, lightgradually leaks into adjacent cores. This phenomena is calleddirectional coupling.

[0063] The amplitude of the j-th layer and the amplitude (A_(j±n)) ofneighboring (j±n)th layer are indicated by the following functions withrespect to z:

A _(j) =E ₀ J ₀(κz)

A_(j±n) =E ₀ J _(n)(iκz)

[0064] where E₀ means the initial amplitude of the input light beam, andJ_(m) indicates the m-th order Bessel function. Symbol “κ” is defined asthe following formula using guided mode W_(j) of the jth layer, W_(j)being defined when the core exists only in the jth layer:

κ=((n _(a) ² −n _(c) ²)λ/(2π))<W _(j) |W _(j−1)>  (2)

[0065] where λ indicates the wavelength, n_(a) means the refractiveindex of the core, n_(c) denotes the refractive index of the cladding,and “<W_(j)|W_(j−1)>” indicates a value obtained by spatiallyintegrating “W_(j)×W_(j−1)” with respect to the (j−1)th core. Here,W*_(j−1) is the complex conjugate of W_(j−1), and W_(j) is a normalizedvalue so that the integration of “W_(j) W*_(j)” over the entire spacebecomes a unity.

[0066] In the present embodiment, the above described strict conditionsare unnecessary, but one condition necessary for reducing the crosstalkis that only contributions to adjacent layers (A_(j±n)) must beconsidered. When “κz” is small, the following relationship is obtained:

A _(j±n) ≈iκz/2

[0067] Therefore, in order to reduce crosstalk due to the directionalcoupling, “κL” must be sufficiently smaller than 1, where L denotes thewaveguide length.

[0068] For example, when n_(a)=1.492, n_(c)=1.480, the thickness of thecore is 1.7 μm, and the thickness of the cladding is 6 μm, κ isapproximately 0.18 m⁻¹. Therefore, the coupling to a neighboring layerat the wave-guided distance of 5 mm is approximately 0.1% in theamplitude base, or approximately 10⁻⁴% in the intensity base. It isobvious that these levels are sufficiently small. That is, when thewave-guided distance is short, such as a few millimeters, crosstalk dueto directional coupling is sufficiently small to be ignored.

[0069] On the other hand, when laser beam 13 is focused on apredetermined reflection point 18, if the difference between therefractive indices of the cladding and the core is small, then thereflection loss in the multi-layers of “cladding/core/cladding/core/ . .. ” has a sufficiently small value to be ignored in comparison with thecoupling loss between the lens and the waveguide.

[0070] In the above example (n_(a)=1.492 and n_(c)=1.480), even if amost undesirable thickness is obtained due to an interference effect,the reflectance for each pair of “cladding/core” is on the order of10⁻⁵. That is, even if a laser beam vertically passes through 100 unitsof the waveguide layers, transmittance of 99% or more can be maintained.The portion having the largest reflection loss is the interface betweenthe air and the uppermost clad layer, and this interface has thereflectance of 3.9% at the most. The above-explained fact that thereflection loss (effect) can be ignored is also applied to thediffracted light, that is, light diffracted towards a direction nearlyperpendicular to the waveguide plane is not much affected by themulti-layers. In addition, if the original diffraction efficiency is setto be low, re-diffraction of the diffracted light, caused by a periodicscattering center existing in another layer, can be sufficiently smallto be ignored. That is, with the original diffraction efficiency η, themaximum quantity of re-diffracted light is η² for each layer. If η=0.5%,then η²=2.5×10⁻⁵. Therefore, even if the most undesirable case, in whichall of the 100 layers have the maximum re-diffraction, is assumed, thediffraction efficiency merely becomes half (2.5×10⁻⁵×100=0.25%).

[0071] The storage capacity depends on the wavelength (λ), the area ofthe card (S), and the number of layers (L). If error correction is notconsidered, the storage capacity is approximately S×L×λ⁻². Therefore,when using a visiting (or credit) card size (S=5.4×9=48.6 cm²) mediumhaving a thickness of 1 mm (L≈100) and a red semiconductor laser (λ=680nm), the storage capacity of 131 GB (giga byte) is obtained. Therefore,even if error correction codes are used and thus the possible capacityfor storing information is decreased, approximately 100 GB can besecured. This capacity is obviously large in comparison with 4.7 GBwhich is the storage capacity of DVDs (Digital Video or Versatile disc).

[0072] Hereinbelow, the periodic scattering center will be explained indetail. First, necessary periodicity will be explained. When thepropagation constant β of the waveguide is indicated by “2π/λ′” (λ′indicates the length corresponding to the period of the guided mode),“Λ” denotes the length corresponding to the period of the scatteringcenter, and λ indicates the light wavelength in vacuum, the angle θbetween the wave-guiding direction and the direction in which thediffracted light proceeds is represented by the following formula (3):

cos θ=λ(1/λ′−1/Λ)   (3)

[0073] Here, if it is assumed that the guided wave travels in the zdirection and that the light confinement is effective in the xdirection, the amplitude of the electric field of the guided wave isrepresented as follows (refer to Amnon Yariv, “Optical Electronics (4thedition)”, ISBN0-03-047444-2, Saunders College Publishing, pp. 479-487):

A(x, y, z, t)=A′(x)exp i(βz−107 t)+A′*(x)exp −i(βz−ωt)

[0074] The propagation constant (β) is defined based on the aboveformula. That is, if the period of the scattering center is the same asthe period of the guided mode, the light is almost perpendicularlydiffracted. If the multi-layered holographic read-only memory cardaccording to the present invention is manufactured using a material suchas a polymeric material, the change of the period of the scatteringcenter due to the thermal expansion, that is, the following formula (4)must be considered.

Λ=Λ₀ +§δT   (4)

[0075] where § indicates the coefficient of linear expansion and δTindicates the change of temperature. If θ≈π/2, then the change δθ ofdiffracted angle is indicated by the following formula (5):

δθ≈−λδT/(Λ sin θ)   (5)

[0076] where the range of § is 10⁻⁴ to 10⁻⁵K⁻¹. Therefore, if δT=±20°C., the change of the diffracted angle is 2×10⁻³ rad (±0.1°) or less,regardless of the size of the relevant hologram. The above change of thediffracted angle is small. However, when the fineness of the hologrampattern is improved, a small change of the diffraction angle may affectthe data retrieval; thus, the retrieval device must be carefullydesigned. For example, if the diffracted light is directly coupled as areal image with the CCD (charge coupled device), each pixel of which hasan area of 5 μm×5 μm, then the distance between the holographic card andthe CCD must be sufficiently smaller than 2.5 mm.

[0077] Here, a simple periodic scattering center may be insufficient forgenerating actual data for desired information. That is, practically, itis necessary to prepare a scattering center which is designed so as togenerate a desired image at the receiving element. When the amplitude ofthe electric field with respect to a desired distribution of lightintensity on the CCD is indicated by E(r_(D)), the amplitude of theelectric field of light guided in the holographic card is indicated byW(r_(W)), and the spatial distribution of scattering intensity of thescattering center is indicated by S(r_(W)), the S(r_(W)) should bepreviously determined so as to satisfy the following formula (6).

E(r _(D))=∫S(r _(W))W(r _(W))exp(i(2π/λ)|r _(W) −r _(D)|)dr _(W)   (6)

[0078] where r_(W) and r_(D) respectively indicate position vectors onthe waveguide plane and the CCD light-receiving plane, and“|r_(W)−r_(D)|” indicates the distance between r_(W) and r_(D). Asdescribed above, the spatial distribution S(r_(W)) is produced using aconcavo-convex distribution, a refractive-index distribution, or thelike, and must be carefully designed so as to generate the necessarydiffracted optical pattern. Here, for convenience of explanation, it isassumed that a plane wave is diffracted towards a directionperpendicular to the waveguide plane, and the necessary concavo-convexpattern and refractive-index distribution will be estimated below. Theconcavo-convex distribution corresponds to a refractive-indexdistribution related to two kinds of refractive indices of the claddingand core; thus, only the refractive-index distribution will be explainedhere.

[0079]FIG. 2 is a diagram explaining the periodic scattering center.

[0080] As shown in FIG. 2, when the variation of the refractive index isindicated by δ₀ and the thickness of the structure related to therefractive-index distribution is indicated by d_(D), the diffractionefficiency (η) is represented by the following formula (7) while thediffraction efficiency is small:

η≈(δ₀ rLd _(D)/(Λd _(a)))²   (7)

[0081] where r indicates the duty of the refractive-index modulation andL indicates the length of the waveguide. If the scattering center isfabricated by providing a concavo-convex pattern in a core, each concaveportion is filled with the material of the cladding; thus, δ₀ is givenby “n_(c)−n_(a)”. With distance x′ between adjacent portions of theperiodic scattering center and width y′ of each portion of thescattering center, the following relationships are defined:

x=x′/Λ

y=y′/Λ

1/r=(½) (1/x+1/y)

[0082] As for a typical example in which n_(a)=1.492, n_(c)=1.480, thethickness of the core d_(a)=1.7 μm, and the wavelength of light δ=680nm, the length λ′ corresponding to the period of the guided mode is 0.46μm. Therefore, if the length (Λ) corresponding to the diffraction periodis adjusted to satisfy the condition “Λ=λ′” and if r=0.1, L=2 mm, andd_(D)=0.05 μm, then the diffraction efficiency is approximately 0.6%.

[0083] On the other hand, if a periodic refractive-index distribution isfabricated by irradiating the core layer with an ultraviolet ray, and ifa waveguide having the structure similar to that in the previous exampleis used and r=0.5, d_(D)=d_(a), L=5 mm, and δ₀=10⁻⁵, then thediffraction efficiency is approximately 0.3%.

[0084] In order to form a master plate for the above concavo-convexinformation pattern, a highly-developed precision technique isnecessary; thus, it is difficult to forge the present memory card incomparison with magnetic cards which can be easily manufactured using amagnetic head.

EXAMPLE 1

[0085]FIG. 3 and FIGS. 4A to 4C are diagrams explaining exampleprocesses of manufacturing the multi-layered holographic read-onlymemory card according to the present invention.

[0086] The present card has a multi-layered structure of a UV curableresin and a PMMA such as “UV curable resin/PMMA/UV curable resin/PMMA/UVcurable resin/ . . . PMMA/UV curable resin”, and the UV curable resinhas the refractive index of 1.480 and the thickness of 8 μm, while thePMMA has the refractive index of 1.492 and the thickness of 1.7 μm.

[0087] In the manufacturing, first, the surface of a 1 inch×1 inchoptically-polished glass substrate 21 is spin-coated with UV curableresin 22 to a thickness of 8 μm, and then the coated surface is exposedto ultraviolet ray 23. After this operation, the coated surface isfurther spin-coated with PMMA 24 to a thickness of 1.7 μm, and roller 25having a concavo-convex pattern, the length corresponding to the periodof which is 0.46 μm, is rolled on the coated surface.

[0088] Next, the 4-step process such as this “coating with UV curableresin/ultraviolet exposure/coating with PMMA/rolling operation” isrepeated 10 cycles, and as the last process, the operation “coating withUV curable resin→ultraviolet exposure” is performed once, therebyobtaining slab waveguide 26 having a periodic structure as shown in FIG.4A. In this case, the UV curable resin layer functions as the claddingwhile the PMMA layer functions as the core, and a concavo-convex patternfunctioning as the periodic scattering center is fabricated in each corelayer.

[0089] After the above operations, one of the edges is polished to aninclination of 45°, as shown in FIG. 4B. A semiconductor laser ofwavelength 680 nm is used as a light source, and the laser beam iscollimated via a collimator lens into a collimated beam having thediameter of 5 mm. The collimated beam is then converged usingcylindrical lens 27 having the focal length of 13 mm.

[0090] As shown in FIG. 4C, the laser beam 126 is focused at the 45°-cutposition of a target PMMA layer so that diffraction light 28 can beobserved in the upward and downward directions.

EXAMPLE 2

[0091] After portions other than the slanted edge of the holographicmemory card manufactured in the above example 1 are covered with analuminum foil, aluminum is vapor-deposited in a vacuum atmosphere. Thealuminum foil is then peeled in the air, thereby forming an aluminumreflecting film on the 45°-cut portion. This 45°-cut portion is furthercoated with a UV curable resin and irradiated with an ultraviolet ray soas to protect the cut portion. As in the above example 1, a beam of thesemiconductor laser of 680 nm is converged onto each PMMA layer by usinga lens having a focal distance of 13 mm, thereby generating thediffracted light in the upward and downward directions.

EXAMPLE 3

[0092]FIGS. 5, 6A to 6C are diagrams explaining processes ofmanufacturing the multi-layered holographic read-only memory card asanother example according to the present-invention, by using thepatterning of a UV curable resin.

[0093] The multi-layered structure of the holographic card of thisexample has a unit consisting of three kinds of UV curable resins suchas “UV-A/UV-B/UV-C/”, and the structure is “unit-1/unit-2/ . . ./unit-n/UV-A”, where n=4 in this example.

[0094] The UV-A has a refractive index of 1.480 and a thickness of 8 μm,the UV-B has a refractive index of 1.492 and a thickness of 1.5 μm, andthe UV-C has a refractive index of 1.475 and a thickness of 0.2 μm.

[0095] In the manufacturing processes as shown in FIG. 5, first, glasssubstrate 31 is spin-coated with UV-A (indicated by reference numeral32-1) to the thickness of 8 μm, and then ultraviolet ray 33 is uniformlyradiated onto the substrate. Next, the above coated surface is furtherspin-coated with UV-B (indicated by reference numeral 32-2) to thethickness of 1 μm, and then the patterning using ultraviolet ray 33 isperformed. Uncured portions are washed and removed using an ethersolvent. Then, the substrate is further spin-coated with UV-C (indicatedby reference numeral 32-3), and the ultraviolet ray is uniformlyradiated onto the substrate. The set of the above processes is repeatedby 4 cycles, and then the set of spin-coating using UV-A (32-1) and theultraviolet exposure is performed once, thereby obtaining slab waveguide34 having a multi-layered structure as shown in FIG. 6A.

[0096] After the above operations, one of the edges is polished to aninclination of 45°, as shown in FIG. 6B. A semiconductor laser ofwavelength 680 nm is used as a light source, and the laser beam iscollimated via a collimator lens into a collimated beam having adiameter of 5 mm. The collimated beam is then converged, by using aplano-convex lens 35 having a focal length of 20 mm, at the 45°-cutposition of each UV-C layer, so that diffraction light 36 can beobserved in the upward and downward directions.

[0097] In the above-described examples, resin is used as a mainmaterial. In this case, mass production using a stamper is possible atlow cost (the stamper cannot be used for the medium based on the volumeholography). However, glass or the like may be used instead of resin.

[0098] The angle of the reflecting surface, at which light is incident,is not limited to 45°, but any angle is possible. That is, any incidentdirection of light can be selected.

[0099]FIG. 7 is a diagram explaining the state of travel of a guidedwave. In the figure, part (a) shows the section of the waveguide and thearrangement for retrieving data, as shown in FIG. 1, and part (b) is aperspective view observed from the topside of the card.

[0100] As shown in the part (b), in the above first embodiment, laserbeam 13, coupled with the slab waveguide by using coupling lens 14,travels and expands into a fan-shape, the pivot (the intersection of thetwo edges of the fan) corresponds to the coupling point of the guidedwave (the reflection point) 18.

[0101] If the refractive index (n_(a)) of each core 12 of the waveguideis 1.492; the refractive index (n_(c)) of each cladding 11 of thewaveguide is 1.480; and the NA of coupling lens 14 (i.e., NA_(L)) agreeswith the NA of the waveguide (NA_(WG)={square root}(n_(a) ²−n_(c) ²))and thus NA_(L)=0.19, then guided wave 17 travels and expands atapproximately 14.6°, as shown in the part (b). As for a single couplingpoint, light only travels within the fan defined by approximately 14.6°.That is, data recorded in the area other than the fan-shaped portioncannot be retrieved and thus that area cannot be efficiently used.

[0102] Data capacity (M) of the hologram of a layer is approximatelygiven by M=S/λ², where S means the area where the hologram exists and λindicates the wavelength to be used, while the data capacity of ahologram is limited according to the number of pixels of thelight-receiving element. Here, it is assumed that λ=680 nm and a CCDhaving 200 million pixels is used. If the hologram corresponds to alight-dark binary data image, then S=0.92 mm², while if the hologramcorresponds to a 8-bit gray scale image, then S=7.4 mm². In each case,it is clear that using an area larger than the above area (S) isredundant. The above calculated area is much smaller than the visiting(or credit) card size (54×90=4860 mm²), and a card having such anextremely small size is rather inconvenient for a portable use. Anotherembodiment for solving the above problem will be shown below.

[0103] Second Embodiment

[0104] In this embodiment, a plurality of fan-shaped portions are mostclosely arranged in a plane, as shown in FIG. 8. In this arrangement, apair of parallel edges 15, 15 of the slab waveguide are cut so as to beslanted by 45° with respect to the waveguide plane, and a plurality ofcoupling points are provided in each edge (as shown by arrows in FIG.8). The structure in which a plurality of fan-shaped portions arealternately combined as shown in FIG. 8 is called a “block” 218,hereinbelow.

[0105] In addition, as shown in the plan (view) of part (a) of FIG. 9,it is possible to manufacture a card in which blocks 218, that is, firstblock 218-1 and second block 218-2, are arranged side by side.

[0106] In this case, the first block 218-1 and second block 218-2 arearranged side by side in the guided direction. Two edges of the firstblock 218-1, the positions of which are indicated by arrows A and B, arecut so as to be slanted by 45°, and the two edges of the second block218-2, the positions of which are indicated by arrows B and C, are alsocut so as to be slanted by 45°.

[0107] In FIG. 9, the coupling points 18 are indicated by “◯”. At theedge indicated by arrow B, the coupling points of the first and secondblocks 218-1 and 218-2 contact each other. Accordingly, the targethologram to be retrieved can be changed by slightly shifting therelative position between the card and the coupling lens.

[0108] The guided wave which starts and travels from the pivot of thefan-shaped portion is reflected by the 45°-cut edge at the side oppositeto the coupling point, and the reflected light goes out of the card andacts as stray light. The positional relationship between the hologramand the light-receiving element to which the holographic image iscoupled must be carefully determined, taking into consideration that thelight-receiving element must not be exposed to such stray light.

[0109] As a method for preventing stray light, the light-receivingelement is positioned at the opposite'side (the lower side in the sideview of part (b) of FIG. 9) to the direction towards which the guidedwave travels after the wave is reflected by the 45°-cut edge (the upwarddirection in the side view of part (b) of FIG. 9). In FIG. 9, referencenumeral 219 indicates a transparent resin.

[0110] In this holographic card, the operation of converging a laserbeam via a lens into a predetermined position is necessary as in theordinary optical disc. In the optical disc, data stored in the disc faceis retrieved one bit at a time, and the target bit is changed bychanging the convergence point. In contrast, in the holographic cardaccording to the present invention, the change of the convergence pointleads to the change of the image (i.e., two-dimensional informationdata). However, the data are retrieved by changing the convergingposition also in this case; thus, similar to the system for the opticaldisc, alignment of the converging position is important in considerationof speed and accuracy of the positioning operation.

[0111] Typically, the size of each coupling point indicated by “◯” inFIG. 9 is approximately 1 mm; thus, positioning must be performed in theaccuracy of approximately 1 μm so as to be able to differentiate eachlayer. Such a positioning technique is practically and widely used atthe response frequency of a few kHz for the ordinary optical disc. Whenthe convergence point exists inside the portion “◯”, it is possible tochose a desired layer (or a desired block if the target coupling pointexists at the edge B) inside the portion “◯” with a response speed ofapproximately 1 ms. Accordingly, if a plurality of coupling lensescorresponding to the number of coupling points “◯” in the card areprovided, any layer in any block can be accessed in approximately 1 ms.

[0112] If the number of coupling points is too large to provide the samenumber of coupling lenses and actuators, then the coupling lenses mustbe reduced (one coupling lens is always necessary). In such a case, inorder to retrieve the holographic image from a desired layer of anyblock, a transfer mechanism must be provided with the holographic card,or a coarse adjustment mechanism for moving the coupling lens in theorder of a few centimeters must be provided.

[0113]FIG. 10 shows an example arrangement using a plurality of blocksand a cylindrical lens as lens 14. In the figure, part (a) shows a planview while part (b) shows a side view. Each of the first block 259-1 andthe second block 259-2 has four rectangular areas, and each couplingportion 260 is provided between two adjacent rectangular areas of thetwo blocks. As shown in part (b), both edges of each rectangle area arecut so as to be slanted by 45°, as in the case of the above-describedfan-shaped arrangement. Also in the present structure, adjacent couplingportions of both blocks contact each other along the edge B; thus, datastored in an area belonging to the other block can be retrieved byslightly changing the convergence position. In addition, the presentarrangement has an advantage that each coupling portion can be much moreeasily positioned and the data retrieval mechanism can be much moreeasily driven in comparison with the fan-shaped arrangement.

[0114]FIGS. 11 and 12 show example arrangements of the holographicmemory card, light-receiving elements, and coupling lenses, in which aplurality of coupling lenses, the number of which is larger than thenumber of blocks by 1, are provided.

[0115]FIG. 11 shows a read-only memory card having two blocks while FIG.12 shows a read-only memory card having three blocks. In each figure,part (a) shows a plan view while part (b) shows a side view.

[0116] Each optical head 221 comprises a laser, a coupling lens, and anactuator, and a light-receiving element 223 such as a CCD is arrangedcorresponding to each head 221.

[0117] If the optical head 221 exists above the edge between the twoblocks, two light-receiving elements 223 are provided for one opticalhead 221. In FIGS. 11 and 12, reference numeral 224 indicates thecoupling point.

[0118] Accordingly, the target layer belonging to any block, from whichthe diffracted light is generated, can be changed simply by shifting theread-only holographic card 222 in a single direction (in the verticaldirection in part (a) of FIGS. 11 and 12).

EXAMPLE 1

[0119] In this example, a semiconductor laser having the wavelength of680 nm is used as a light source, a UV curable resin having therefractive index of 1.480 and the thickness of 9.3 μm is used for eachclad layer, and a PMMA having the refractive index of 1.492 and thethickness of 1.7 μm is used for each core layer.

[0120] In the multi-layered holographic memory card having the size D:9.5 mm×W: 10 mm×h: 0.55 mm as shown in FIG. 13, opposite edges 15 and 15are cut so as to be slanted by 45°, and coupling points are provided atintervals of t: 2.5 mm on each edge, so that the guided wave input fromeach coupling point does not overlap the other guided wave input fromanother coupling point in the waveguide plane.

[0121] In FIG. 13, no guided wave reaches the shaded or hatched areas,that is, these areas do not participate in the data retrieval of thehologram. The remaining triangular areas, which the guided waves inputfrom each coupling point 18 (indicated by bold “black” arrows) occupy,do not overlap with each other.

[0122] When light was incident via a lens of a diameter of 5 mm and afocal length of 15 mm from different coupling points of the same layerof the holographic card having the above structure, differentholographic images were retrieved while these images did not overlapwith each other.

EXAMPLE 2

[0123] As shown in FIG. 14, two blocks (218-1 and 218-2) are arrangedside by side, each block corresponding to the multi-layered holographicmemory card having the size D: 9.5 mm×W: 10 mm×h: 0.55 mm as shown inexample 1. The two blocks are covered and strengthened using epoxy resin219 so as to make a holographic card 200. Gold is vapor-deposited on the45°-cut edges of each block to approximately 200 nm (thickness). Inorder to improve the mechanical strength of the card, the area coveredby epoxy resin 219 is larger than the total size of the two blocks, andthe card size is 25 mm×15 mm×0.55 mm.

[0124] When light was incident via a lens of a diameter of 5 mm and afocal length of 15 mm from different coupling points of the same layer(of the same block) in the card having the above structure, differentholographic images are retrieved.

[0125] The arrangement according to the above second embodiment has manycoupling points and the interval thereof is a few millimeters.Therefore, if a retrieval device having a single head including a lightsource and a two-dimensional detector is used, the head must be linearlymoved so as to change the coupling point to be accessed. The structureof the mechanical device for linear movement with a high accuracy isgenerally complicated and the operation speed thereof is limited; thus,the performance of the device affects the data-retrieval speed. Theproblem of the access speed can be solved by employing many heads in theretrieval device; however, the structure of the device is verycomplicated and the required cost is higher in this case.

[0126] In order to solve the above problem that either many heads mustbe provided or the card must be linearly moved, the following embodimentwill be shown, by which necessary linear motions are reduced and theaccess speed is greatly improved by using only a single head, and thestructure can also be simplified.

[0127] Third Embodiment

[0128]FIG. 15 is a diagram explaining the disc-shaped memory(abbreviated to “disc memory”, hereinbelow) and the data retrievalmethod in the present embodiment. FIG. 16 is a diagram explaining theunit for storing data in the memory of the present embodiment.

[0129] In these figures, reference numeral 301 indicates a laser beam,reference numeral 304 indicates guided waves, reference numeral 306indicates a coupling point, reference numeral 310 indicates aholographic image, reference numeral 311 indicates a disc memory,reference numeral 312 indicates a light source, reference numeral 313indicates a two-dimensional detector, reference numeral 314 indicates a45°-slanted conical (i.e., cone-shaped) reflecting surface, referencenumeral 315 indicates a sector, reference numeral 316 indicates astratum, and reference numeral 317 indicates a disc.

[0130] The disc memory 311 according to the present invention has adoughnut shape, as shown in FIG. 15. In order to provide coupling points306 with laser beam 301. a central portion of the disc is opened so asto produce a 45°-slanted conical hole. The 45°-slanted conical surfaceis polished so as to provide reflecting surface 314.

[0131] As shown in FIG. 15, the radius of each fan-shaped area where thehologram is recorded is a little smaller than the real radius of thedisc memory 311. Therefore, a “gap” area of the data distribution isgenerated between two adjacent fan-shaped areas, and the effectivedata-storage density is decreased. The smaller the difference betweenthe radii of the fan-shaped area and the disc, the smaller the ratio ofthe area of such gaps to the total disc area. That is, the shorter thedistance between the coupling point 306 and the center of the disc, thebetter the data-storage density. Therefore, preferably, the center holehas the smallest possible radius.

[0132] Here, the means for coupling light from the outside to thewaveguide is not limited to the above-explained 45°-slanted reflectingmirror surface.

[0133] Hereinbelow, a fan-shaped portion having the multi-layeredstructure is called “sector” 315, as in the magnetic disc. A pluralityof sectors 315 having the same layers and corresponding to one rotationare called stratum 316. Furthermore, a set of strata 316 is called adisc 317.

[0134] The relationship between sector 315, stratum 316, and disc 317 isshown in FIG. 16.

[0135] Data fully stored in a sector 315 can be retrieved using one ormore two-dimensional detectors at a time, and data fully stored in astratum 316 can be retrieved by rotating the disc memory 311 by oneround while the position of light source 312 is fixed. In addition, datafully stored in the whole disc memory 311 are included in disc 317.These relationships are shown in the diagram of FIG. 16.

[0136] In the method of manufacturing the medium, the generation of themulti-layered structure and fabrication of the hologram in a targetlayer are similar to those in the above-explained first embodiment. Inorder to complete the whole memory in the present embodiment, fan-shapedportions, each having multi-layered sectors, may be individuallyproduced, and these portions are circularly arranged and bonded witheach other. As an easier method, disc-shaped strata are first produced,and they are stacked to be multi-layered. In this case, after thewaveguides are multi-layered, a center hole is provided and the openedportion is polished so as to have a conical shape. Coupling points arethen provided in the polished surface. The polished surface itselffunctions as a total reflection mirror while the surface is exposed inthe air. However, a metal film made of aluminum, gold, or the like maybe vapor-deposited onto the reflecting surface.

[0137] The data retrieval is performed as shown in FIG. 15.

[0138] A light source is positioned immediately above the 45°-slantedconical reflecting surface 314 on which coupling points 306 to themedium are provided. Incident light is coupled to one of themulti-layered waveguides, and travels towards the outer circumference ofthe disc memory while the light is confined inside the waveguide. Duringthis operation, the hologram provided in the waveguide (as explainedabove) diffracts the light towards the upward and downward directions sothat holographic image 310 is generated. This image is detected usingtwo-dimensional detector 313 (typically, a CCD) so that an electricsignal representing the stored data can be obtained. The convergenceposition of the light from light source 312 should be shifted when atarget layer from which data are retrieved is changed. Therefore, amechanism for performing a micro-motion of convex lens 302 is necessary.The actuator used for a micro-motion of the head of an optical discdrive may be used as the above mechanism. Only a single two-dimensionaldetector 313 is shown in FIG. 15; however, a plurality oftwo-dimensional detectors may be used. Similarly, a plurality of lightsources may be used for improving the access speed.

[0139] In addition, the method of rotating the disc memory is notlimited, and there are a plurality of methods for selecting a targetsector. For example, the disc is usually stopped, and only when dataretrieval is performed, the disc is moved and rotated using a servomotor so that the target coupling point is positioned at the lightsource. However, in consideration of the access speed, it is preferablefor the disc to always rotate with a high speed, as in the cases of theCD and the hard disc. In this case, the method of selecting the sectoris important, and the method used for the conventional CD, magneticdisc, and the like can be easily applied to the present memory. Forexample, a synchronous signal may be included in the hologram, or aphysical mark provided in the medium. These methods have been applied tothe conventional magnetic discs. In addition, a remarkable merit as forthe disc-shaped memory is the leakage of light of the guided wave fromthe outer circumference of the disc. As described above, in themulti-layered holographic read-only memory card, it is necessary to havea low diffraction efficiency with respect to the hologram of each layer,so as to depress crosstalk between adjacent layers. Accordingly, mostparts of the guided wave are absorbed or scattered at the outercircumference of the fan-shaped portion, or transmitted to the outside.Therefore, there is a synchronous method of extracting and using such anon-diffracted portion of the guided wave. This extracted portion of theguided wave has a power much higher than that of the holographic image;thus, it is much more advantageous than using a hologram which itselfincludes a synchronous signal.

EXAMPLE 1

[0140]FIG. 17 shows a disc memory 400 manufactured according to thepresent invention.

[0141] As shown in the figure, in this memory, the waveguides aremulti-layered on glass substrate 319. Reference numeral A indicates asectional view of the part indicated by a dashed line. This memory has acentral hole 400 a having the diameter of 1 mm, and the edge 314 of themulti-layered waveguide 318 has a 45°-slanted conical shape. A hologramis fabricated in a shaded part of the diameter of 22 mm in the disc.Here, a stratum consists of 24 sectors, and 10 layers are deposited. Alltogether, data corresponding to 10 strata and 240 sectors are recorded.

[0142] The manufacturing method was as follows. First, 10 plates of themetal mold having a concavo-convex pattern of a hologram were prepared.One plate of the metal mold corresponds to one stratum. Next, on a glassdisc 319 having the diameter of 23 mm, a plurality of waveguides weremulti-layered, the cladding being made of a UV curable resin having therefractive index of 1.480 while the core being made of a PMMA having therefractive index of 1.492. In the depositing operation, the substratewas first spin-coated with the UV curable resin and further spin-coatedwith the PMMA, and an ultraviolet ray was uniformly radiated. Then, theplate of the metal mold for the first stratum was pushed onto the coatedsurface so as to transfer the hologram. The above series of operationssuch as spin-coating of the UV curable resin, spin-coating of the PMMA,radiation of the ultraviolet ray, and transfer of the hologram for thetarget stratum was repeatedly performed. Finally, one more clad layerwas deposited so as to cover the whole portion, and a multi-layeredoptical waveguide comprising 10 layers could be obtained. The averagethickness of each core layer was 1.7 μm while the thickness of eachcladding was 9.3 μm.

[0143] After the above processes, a hole having the diameter of 1 mm wasprovided in a central area, as shown in FIG. 17, the hole surfacecorresponding to waveguide layers was processed to have a 45°-slantedconical shape and was polished using a diamond saw. Subsequently,aluminium was vapor-deposited onto the slanted portion so as to producean aluminium reflecting film. The reflecting film was covered using aresin, and then an attachment for rotating the disc was equipped.

[0144] The memory (of the diameter of 23 mm) storing image data of 10strata and 240 sectors was obtained via the above-explained processes.

[0145] The image data stored in the above-manufactured disc memory wereretrieved using the system shown in FIG. 15. However, FIG. 15 is notillustrated in proportion, and the dimensional ratio of each part doesnot always correspond to the real ratio.

[0146] A collimated semiconductor laser beam of the wavelength of 680 nmwas converged by a lens having a diameter of 5 mm and a focal length of13 mm, at the slanted portion of the waveguide layer, that is, at thetarget coupling point. The lens was moved in the upward, downward, leftand right directions by using a micro-motion. actuator so that theconvergence point agreed with the coupling point of the first layercounted from the surface of the medium. Accordingly, an image recordedin one of the 240 sectors in the memory was retrieved.

[0147] While the disc memory was finely rotated, a rotation angle atwhich the image was most clearly retrieved was found. At this angle,accurate image data could be retrieved using the two-dimensionaldetector (here, a CCD).

[0148] When the disc memory was further rotated by approximately 15°, animage other than the above-retrieved image was retrieved, that is, datastored in the next sector could be retrieved. After that, every time thedisc memory was further rotated by approximately 15°, another image wasretrieved. According to the above operations, data stored in 24 sectors,that is, data of one stratum could be retrieved.

[0149] Similar operations ware performed after the convergence point ofthe laser beam was shifted to the coupling point of the second layer,thereby retrieving data stored in the second stratum. Every time thecoupling point (i.e., target layer) was changed, data of one stratumwere retrieved. All together, image data stored in 240 sectors of 10strata were accurately retrieved via 10 coupling points.

EXAMPLE 2

[0150] As shown in FIG. 18, synchronous mask sequence 321 was providedin the side face 320 of the disc memory. In this mask sequence, a binarybit indicates whether light is transmitted, and a set of 32 bits (i.e.,2 words) is provided. Each set corresponded to a fan-shaped portionhaving 10 multi-layered sectors, and was placed at the arc of therelevant fan-shaped portion.

[0151] Accordingly, synchronous mask sequence 321 had a total of 32×24bits (i.e., 2×24 words). Among 32 bits corresponding to one fan-shapedportion, the upper 10 bits were assigned to signal synchronization,while the lower 5 bits were assigned to sector identification. The 10bits for signal synchronization has a bit sequence “0101010101” for all24 sets. In addition, numbers from 0 to 23 were respectively assigned tothe above 24 fan-shaped portions, and these numbers were reflected inthe 5 bits for sector identification of the synchronous mask sequence321 provided to the target fan-shaped portion. A laser beam was inputand coupled to a sector of the disc memory, to which the synchronousmask sequence 321 was added as described above, by using a dataretrieval system as shown in FIG. 15.

[0152] As explained above, the largest part of the guided wave 304 wasleaked from the side face 320 of the disc memory to the outside; thus,optical pattern 322 corresponding to the synchronous mask sequence 321was observed near the side face of the disc memory, and was detectedusing one-dimensional detector 323 so that the pattern of thesynchronous mask sequence 321 could be detected.

[0153] Here, the signal of the one-dimensional detector 323 wasmonitored while the stationary rotation at approximately 70 rpm of thedisc memory was performed. According to the monitoring operation, it wasconfirmed that the upper 10 bits for signal synchronization of thesynchronous mask sequence 321 were “0101010101”. Simultaneously, thelower 5 bits were detected and the image data are detected by thetwo-dimensional detector 313 so that data stored in a target sector andthe number assigned to the fan-shaped portion to which the sectorbelongs could be obtained.

[0154] According to the above method, even when the stationary rotationwas performed, data of the designated sector could be retrieved. Therotation speed of the disc memory was approximately 70 rpm, as describedabove; thus, all data stored in the disc memory were retrieved inapproximately 8.7 sec at the quickest.

What is claimed is:
 1. A multi-layered holographic read-only memory inwhich single-mode slab waveguides are stacked to be multi-layered,wherein: a periodic scattering center whose period approximately agreeswith the period of the guided mode is provided in at least one of a corelayer and a clad layer in each waveguide so that a guided wave in thewaveguide is diffracted by the periodic scattering center to the outsideof the waveguide and a holographic image is generated.
 2. Amulti-layered holographic read-only memory as claimed in claim 1,wherein at least one of the core layer and the clad layer is made of aUV curable resin and a hologram is recorded by the ultraviolet patternirradiation, and the recorded hologram functions as the periodicscattering center.
 3. A multi-layered holographic read-only memory asclaimed in claim 1, wherein master data including a concavo-convexpattern are transferred to at least one of the core layer and the cladlayer so that the concavo-convex pattern is fabricated, and thefabricated concavo-convex pattern functions as the periodic scatteringcenter.
 4. A multi-layered holographic read-only memory as claimed inany one of claims 1-3, wherein at least one of the edges of themulti-layered slab waveguide is cut so as to produce a reflectingsurface which is slanted by a predetermined angle with respect to thenormal direction of the waveguide plane.
 5. A multi-layered holographicread-only memory as claimed in claim 4, wherein the predetermined angleis approximately 45°, and light is incident from the directionsubstantially perpendicular to the waveguide plane on the reflectingsurface so as to introduce the light into the waveguide.
 6. Amulti-layered holographic read-only memory as claimed in claim 4,wherein the reflecting surface is covered with a metallic or dielectricreflecting film.
 7. A multi-layered holographic read-only memory asclaimed in claim 5, wherein the multi-layered slab waveguide hasopposite edges which function as 45°-cut reflecting surfaces, and guidedwaves incident from these edges do not overlap with each other in therelevant waveguide plane.
 8. A multi-layered holographic read-onlymemory as claimed in any one of claims 1-3, wherein a plurality of themulti-layered slab waveguides are placed and bonded with each other inthe waveguide plane so as to make a card.
 9. A multi-layered holographicread-only memory as claimed in claim 7, wherein a plurality of themulti-layered slab waveguides are placed and bonded with each other inthe waveguide plane so as to make a card.
 10. A multi-layeredholographic read-only memory as claimed in any one of claims 1-3,wherein the multi-layered slab waveguide has a disc shape, and alight-introducing section is provided in a central area of the disc, soas to guide light towards the outer circumference of the disc.
 11. Amulti-layered holographic read-only memory as claimed in claim 10,wherein: the light-introducing section is a reflecting surface havingthe shape of a 45°-slanted side face of a cone, and light is incidentfrom the direction substantially perpendicular to the disc plane on thereflecting surface so as to introduce the light into the waveguide. 12.A multi-layered holographic read-only memory as claimed in claim 11,wherein: a plurality of coupling points for introducing light areconcentrically and periodically placed in the reflecting surface; aguided wave is transmitted from each coupling point towards the outercircumference of the disc while the guided wave expands as a fan-shapehaving a predetermined angle of expansion; and the predetermined angleof expansion is determined in order that fan-shaped portionscorresponding to each coupling point do not overlap with each other. 13.A multi-layered holographic read-only memory as claimed in claim 12,wherein a specific identification pattern for each coupling point isprovided in an area of the side face of the disc, the guided wave fromthe coupling point being transmitted through said area.
 14. A method forretrieving data stored in the disc-shaped memory as claimed in claim 10by rotating the memory, the method comprising the step of extracting anon-diffracted portion of the guided wave outside of the memory so as toestablish a synchronous condition in the rotation.
 15. A method forretrieving data stored in the disc-shaped memory as claimed in claim 12by rotating the memory, the method comprising the steps of: providing aspecific identification pattern for each coupling point at an area ofthe side face of the disc, the guided wave from the coupling point beingtransmitted through said area; and detecting the identification patternso as to establish a synchronous condition with respect to the relevantcoupling point.